Metal chloride batteries, especially sodium-metal chloride batteries with a molten sodium negative electrode (usually referred to as the anode) and a beta-alumina solid electrolyte, are of considerable interest for energy storage applications. In addition to the anode, each cell of the battery includes a positive electrode (usually referred to as the cathode) that supplies/receives electrons during the charge/discharge of the battery. The solid electrolyte—often in the form of a tube—functions as the membrane or “separator” between the anode and the cathode. The cathode composition is typically contained inside the electrolyte tube, which is usually open on one end, and closed on the other end.
FIG. 1 is a simple illustration of an energy storage cell 10 (i.e., an electrochemical cell), e.g., one of a group of cells that collectively would form a metal chloride battery. The cell includes a housing 12. The housing includes a separator 14, having an outer surface 16, and an inner surface 18. The outer surface defines a first chamber 21 and the inner surface defines a second chamber 22 The first chamber is usually an anode including sodium, and the second chamber is a usually a cathode that can include a number of salts. The first chamber is in ionic communication with the second chamber through the separator. The first chamber and the second chamber further include an anode current collector 24 and a cathode current collector 26 to collect the current produced by the electrochemical cell. (The particular location and form of the current collectors can vary considerably). Other details regarding such a cell are provided, for example, in U.S. Pat. No. 7,632,604 (Iacovangelo et al), incorporated herein by reference.
The metal chloride batteries and other types of sodium-based thermal batteries can be employed in a number of applications, e.g., as part of the public utility-energy infrastructure. Several specific examples for the batteries include uninterruptable power supply (UPS) devices; and components for a battery backup system for a telecommunications (“telecom”) device, sometimes referred to as a telecommunication battery backup system (TBS). The batteries are often capable of providing power surges (high currents) during the discharge cycle. In an ideal situation, the battery power can be achieved without a significant loss in the working capacity and the cycle life of the battery. The advantageous features of these types of batteries provide opportunities for applications in a number of other end use areas as well.
As alluded to above, the present design of a battery cell like those based on Na—NiCl2 entails having the open end of a beta-alumina solid electrolyte tube joined to an alpha-alumina collar using a glass seal. The collar is in turn joined to nickel rings, with the help of thermal compression bonding (TCB). TCB is achieved through metallizing the alpha-alumina collar. The design of the present cell requires the seal to be resistant towards molten sodium and molten halide. (Sodium melts at 98° C., and NaAlCl4 melts at 157° C.). The glass seal and TCB are two of the weak links in the present design for a path to long life. The glass seal and TCB encounter corrosion from sodium and halide and, because of this, are found to degrade over time.
Different paths have been taken to address the problem of corrosion. One involved trying to improve the quality of the glass seal and TCB. Another was to eliminate the glass seal and the TCB in the design of the cell. As an example, the seal can be eliminated by using a graded ceramic (beta-alumina tube with alpha-alumina header) tube. However, in the design where this graded tube is used, the nickel ring cannot be joined with the alpha-alumina collar using a TCB-like process. Therefore, alternate joining technologies are necessary.
Active brazing is a procedure in which one of the components in a braze alloy composition reacts with a ceramic material and forms an interfacial bond. With the concerns noted above, the braze alloy must be suitable for use in high temperature rechargeable batteries, and be very resistant to corrosion from sodium and halide materials. Very few commercially-available braze alloys possess the high-temperature capabilities required for manufacturing sodium metal halide cells, while also possessing the required corrosion resistance.
Conventionally, brazing is done through metallization, in combination with a braze alloy. The metallization (for example with Mo) is typically carried out at a temperature of about 1550° C. Metallization is a very sensitive process, and depends on a number of variables which need to be carefully controlled to obtain a robust metallization layer. Moreover, a metallization/TCB process can be complicated and expensive. Therefore, it is important to develop new techniques that can replace conventional metallization processes.
Active brazing has been known in the literature to join ceramic to metal, but there are not many commercially-available active braze alloys (ABAs), particularly high temperature (900-1250° C.) ABAs, which are resistant to corrosion from sodium and halide. Recently, certain active braze compositions have been successfully used for the sealing structures in metal halide cells (e.g., for the sodium-based types of batteries). Some of those compositions are described below. However, while their use can often satisfy the rigorous hermeticity requirements for the storage cells, there may still be some deficiencies when the cells are intended for certain applications (though not others). For example, the strength of the joint formed with the active braze composition may not always meet end use requirements.
It is thought that a key factor in diminished strength relates to the thermal expansion mismatch between the parts being brazed, e.g., a ceramic collar and a nickel ring. The relatively large difference in the coefficient of thermal expansion (CTE) can lead to large residual tensile stress in the ceramic component, e.g., an alumina component. The tensile stress can lead to micro-cracking or cracking in the joint, which can in turn lead to leakage and cell failure. This problem is exacerbated by the high temperatures required for some of the brazing compositions, as compared to the temperatures for TCB techniques which have been used for a similar purpose.
With the considerations noted above, new types of sealing structures and compositions for energy storage devices and other types of electrochemical cells would be welcome in the art. The new technology should provide hermetic sealing with a joint strength sufficient to meet rigorous end use requirements for the cell. Moreover, the overall sealing structure should be compatible with electrochemical cell contents that might come into contact with the seals. It would also be desirable if the sealing structures can be obtained with lower fabrication costs, e.g., as compared to some of the metallization/TCB processes used in conventional situations.